The Steam icon is provided up to a size of 48x48px — from the looks of it, this icon is straight from the Windows client. This results in a fuzzy Steam icon in many cases: the dash in Unity; the Launcher in Unity if configured to be larger than the default; essentially anywhere in GNOME Shell; the Ubuntu Software Centre (if / when the package shows a Steam icon). Linux desktop software generally assumes that application icons are available at large sizes.

Changes in the newest client have overwritten the ability to use a custom icon for steam.This picture shows what it should look like with my Faenza Icon Pack for Gnome (steam.svg) and how it looks like. It's not a dealbreaker but it would be nice to have it back, because some people like a consistent appearance. The icon is in /usr/share/icons/Faenza/Apps/steam.svg and has succesfully overwritten the icon provided by Valve automatically since the release of the first client without me ever doing anything.